Frame structure are known which employ webs made from tubular metal having opposed flattened end portions. Fasteners such as bolts or screws are employed at the frame joint to secure the flattened end portions to the chords.
Known frame joints vary in complexity and strength. In Australian patent No. 295,018 a frame structure is described having two chords which extend generally in the same longitudinal direction and a plurality of webs extend in zig-zag configuration in the space between the chords. Each web has opposed flattened end portions which extend along respective inner sides of the respective chords so that adjacent flattened end portions of adjacent webs extend in overlapping relation. A single bolt extends through the overlapping end portions and through the inner side of the chord to form the frame joint by which the adjacent webs are secured to the chords. All the frame joints employed in the frame structure of AU 295,018 are of this type. As a consequence the frame structure can be manufactured from a plurality of identical webs.
A major disadvantage of this frame joint is that it is inherently weak and as a consequence the whole frame structure has a tendency to twist under load. In addition it is often desirable to fix brackets to the frame structure at the frame joint and the frame joint of AU 295,018 does not readily facilitate rigid fixing of a bracket.
In U.S. Pat. No. 3,925,951 a similar frame structure is described. However, in this frame structure two fasteners are employed at each frame joint. The frame structure of U.S. Pat. No. 3,925,951 includes two wooden chords which extend generally in the same longitudinal direction and a plurality of tubular metal webs extend in zig-zag configuration in the space between the chords. Two different webs are employed and alternate along the structure. One web is referred to as a tension web and has opposed flattened end portions. The opposed flattened end portions of the tension web each have two holes, spaced longitudinally relative to the chords, through which the tension web is secured to the chords by fasteners. The other web is referred to as a compression web and has opposed flattened end portions which are arranged in overlapping relation with the flattened end portions of the tension webs. The flattened end portions of the compression webs have a single hole and are secured in overlapping relation with the tension web by a common fastener. Thus each frame joint employs two fasteners only one of which is common to both the compression and tension webs.
While the frame joint of U.S. Pat. No. 3,925,951 is stronger than the frame joint of AU 295,018 the longitudinal spacing of the fasteners provides little transverse strength to resist twisting. In U.S. Pat. No. 3,925,951 two different webs are required which results in increased manufacturing costs.
A further problem arises when adapting the structure of U.S. Pat. No. 3,925,951 to a completely metal frame structure where the chord holes are to be predrilled or prepunched. The fastening holes are in pairs with each hole in the pair closely spaced in the longitudinal direction. Thus the tolerance required in scheduling the holes is much higher than in the case of longitudinally widely spaced holes. This results in the need for higher quality control and associated with higher quality control one can expect higher costs.